1. Field of Invention
The process of the present invention is an improved vacuum freezing process that is useful in separating solvent from a solution that contains one or more non-volatile solutes. It can be used in desalination of sea water and brackish water, renovation of waste water, and concentration of aqueous and non-aqueous solutions.
The major difficulties that a conventional vacuum freezing process has suffered from are related to the way the low pressure water vapor formed in a vacuum freezing zone is removed from the zone and the way it is transformed into a liquid state. The present invention introduces a new and convenient method of accomplishing these without using a compressor to compress the vapor, without using an absorbing solution to absorb the vapor and without pressurizing ice. In the process, the low pressure vapor is desublimed without pressurization and the desublimate is then melted and vaporized to form a vapor that is at a pressure higher than the triple point pressure. The process is highly energy efficient and can be operated simply and reliably.
2. Brief Description of the Prior Art
Several vacuum freezing processes have been introduced by workers in the desalination field. These processes are (1) Vacuum-Freezing Vapor-Compression (VFVC) Process developed by Colt Industries, (2) Vacuum-Freezing Vapor Absorption (VFVA) Process developed by Carrier Coporation, (3) Vacuum-Freezing Ejector-Absorption (VFEA) Process developed by Colt Industries, (4) Vacuum-Freezing Solid-Condensation (VFSC) Process developed in the Catholic University of America (5) Absorption Freezing Vapor Compression (AFVC) Process introduced by Concentration Specialists, Inc., and (b) Vacuum Freezing High Pressure Ice Melting (VFPIM) Process introduced by Chen-Yen Cheng and Sing-Wang Cheng.
In any of the vacuum freezing processes described, an aqueous solution is introduced into a chamber which is maintained at a pressure that is somewhat lower than the vapor pressure of the solution at the freezing temperature of the solution to thereby simultaneously flash vaporize water and form ice crystals. As the results of this operation, a low pressure water vapor and an ice-mother liquor slurry are formed. In case of sea water desalination, this pressure is around 3.5 Torr. The low pressure water vapor formed has to be removed and transformed into a condensed state; the ice crystals have to be separated from the mother liquor and the resulting purified ice has to be melted to yield fresh water. Furthermore, the heat released in transforming the vapor into a condensed state has to be utilized in supplying the heat needed in melting the ice. The processes described utilize different ways of vapor removal, different ways of transforming the vapor into condensed states and different ways of accomplishing the heat reuse.
The Vacuum Freezing Vapor Compression Process is described in the Office of Saline Water, Research and Development Report No. 295. In the process, the low pressure water vapor is compressed to a pressure higher than the triple point pressure of water (4.58 Torr) and is then brought in direct contact with purified ice to thereby simultaneously condense the water vapor and melt the ice. The main disadvantages of this process are that the special compressor designed to compress the low pressure water vapor can not be operated reliably and the compressor efficiency is low.
The Vacuum Freezing Vapor Absorption Process was developed by Carrier Corporation up to 1964, but has been discontinued. The process is described in the Office of Saline Water, Research and Development Report No. 113. In the process, the low pressure water vapor is absorbed by a concentrated lithium bromide solution. The diluted solution is reconcentrated by evaporation and the water vapor so formed is condensed to become fresh water. Heat of absorption is removed by a recycling water stream through a heat transfer surface; the recycling water stream is then used to melt the ice crystals.
The Vacuum Freezing Ejector absorption Process was also developed by Colt Industries and is described in Office of Saline Water, Research and Development Report No. 744. In the process, the low pressure water vapor obtained in the freezing step is compressed by a combination of steam ejector and absorber loop. A concentrated sodium hydroxide solution is used to absorb a part of the low pressure vapor, the diluted sodium hydroxide solution is boiled to form water vapor at 300 Torr and regenerate the concentrated solution. In the ejector the water vapor at 300 Torr is used to compress the remaining low pressure water vapor.
The Vacuum-Freezing Solid-Condensation Process was developed by Professors H. M. Curran and C. P. Howard of the Catholic University of America and is described in Office of Saline Water, Research and Development Report No. 511. In the process, Freon-12 is used to remove the latent heat released in transforming the low pressure vapor into ice and supply the latent heat needed in the melting of both the ice formed in the freezing step and ice transformed from the low pressure water vapor.
The Absorption Freezing Vapor Compression (AFVC) Process has been recently introduced by Concentration Specialists, Inc., Andover, Mass. and a 25,000 gpd pilot plant has been built in OWRT (Office of Water Research and Technology) Wrightsville Beach Test Station. The absorption freezing vapor compression (FAVC) Process is a vacuum freezing process in which the freezing is accomplished in a stirred tank crystallizer due to the evaporation of water vapor which in turn is absorbed in an adjacent chamber by a concentrated solution of sodium chloride (NaCl). The NaCl solution, diluted by the water vapor, is pumped to a generator where it is concentrated to its original strength by a vapor compression cycle using a closed circuit refrigerant as the working fluid. The vapor compression cycle operates between the absorber and generator, taking the heat that is associated with absorption and pumping it up to a level such that it can be used to evaporate the absorbate in the generator. The vapor liberated in the generator is used to melt the ice in direct contact.
In the Improved Vacuum-Freezing High Pressure Ice Melting Process of U.S. Pat. No. 4,236,382, an aqueous solution is flash vaporized under a reduced pressure to simultaneously form a low pressure water vapor and ice crystals. The ice formed is first purified in a counter-washer and then melted inside of heat conductive conduits under a high pressure (e.g. 600 atm.) and the low pressure water vapor is desublimed to form desublimate (ice) on the outside of the conduits. The latent heat of desublimation released is utilized in supplying the heat needed in the ice-melting operation. The desublimate is removed intermittently by an in-situ dissolution operation utilizing an aqueous solution such as the feed solution or the concentrate; about an equivalent amount amount of ice is formed inside of the conduits by an exchange freezing operation. The ice so formed is also melted by the high pressure ice melting operation described.